This invention relates to a nonvolatile semiconductor memory, typically an EEPROM (Electrically Erasable Programmable Read-Only Memory), including a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) and, in particular, to a method of predicting the life of a nonvolatile semiconductor memory as a predicted life and determining operating conditions for the nonvolatile semiconductor memory so that the predicted life achieves an objective value.
From a practical standpoint, it is required in manufacture of VLSI (very-large-scale integrated) circuits that manufactured products can perform their functions throughout their intended useful life, in other words, that the manufactured products are excellent in reliability. In order to achieve an excellent reliability of the products, attempts have been primarily made to improve a manufacturing process. In addition, it is important for a manufacturer to better predict the reliability of his products. In particular, when the reliability is deteriorated due to a particular type of reliability failure mechanism, it is critical for the manufacturer to detect the reliability failure mechanism.
It is believed that the deterioration in reliability of an EEPROM results from two major reliability failure mechanisms. The first mechanism is responsible for early reliability failures and involves submicroscopic defects present in a gate dielectric film (may be referred to as either a tunnel film or a tunnel dielectric film) of the EEPROM. The second mechanism is responsible for extensive reliability failures and involves defects or damages produced in the tunnel film after operation of the EEPROM. A so-called device life depends upon the second mechanism. As will later be described in detail, electrons pass through the tunnel film during operation of the EEPROM. The passage of the electrons causes the defects or the damages to occur in the tunnel film. The presence of the defects or the damages in the tunnel film will result in fluctuation of threshold levels of the EEPROM. Under the circumstances, it is a strong demand in view of the reliability of the tunnel film to predict in a factory stage post-operation characteristics of the EEPROM after operation and to assure the reliability of the products.
In the past, once a product had been completely manufactured, no way existed of predicting the life of the product. It has been a general practice to use screening procedures in which actual products are repeatedly tested to eliminate those products judged as failed products. However, the screening procedures are disadvantageous in that all the products are equally deteriorated through screening tests and that the life of good products surviving the screening tests is not known at all. In order to solve the above-mentioned problems, a proposal is made of a method of predicting a product life and is disclosed in Japanese Unexamined Patent Publication No. 195060/1991. In this method, measurement is made of the erase speed of ICs. The percentage of erased bits is plotted as a function of the applied gate threshold voltage. The percentage of bits located in an erase tail region of distribution is then identified. The bits located in the erase tail region are erased at a relatively high speed as compared with the remaining bits in an array. If the number of bits in the erase tail region exceeds a certain percentage of the total bits in the array, the IC is classified as one which is likely to suffer early endurance failure. Specifically, in the above-mentioned Japanese Unexamined Patent Publication No. 195060/1991, if the number of bits in the erase tail region of a device exceeds approximately 6% of the total bits in the array, the device is downgraded as less reliable. According to this method, it is possible to relatively easily and quickly carry out the screening tests as compared with known methods and to prevent deterioration through the screening tests.
In the above-mentioned conventional technique, however, the screening tests rely upon the early failures. In other words, the screening tests are to eliminate failed products very rapidly deteriorated. This technique is insufficient with respect to prediction of the life and the reliability of "good products" surviving the screening tests.
Furthermore, the prediction of the life in the above-mentioned technique can not be carried out until the device is completely manufactured. If the life then predicted does not reach the intended life, all products having been completely manufactured can not be shipped or delivered. Such problem will be avoided if the prediction of the life is carried out in an early stage of a manufacturing process of EEPROMs.